0) First stop for N900 Tips: N900 Shortcuts And Gestures
1) You can actually hang up (silence) an incoming call by hanging up (turning the phone upside down) (wongdong)
2) You can open the keyboard, press a button and it brings up contacts beginning with that letter (wongdong)
3) You can use the built in jabber IM functionality to use the facebook chat (wongdong)
4) Double tap the power button to lock the device (RafaLL92)
5) Web pages or in messages swipe into the screen from the left and a little mouse pointer appears, tap the button and you can select/copy text on web page and messages (fraaaaanka) This also lets you scroll the map in Google maps. Without doing this, there's no way to scroll a Google map, and screen swipes move the screen around instead of the map (JCH)
6) On home screen you can just start typing to look up contacts (quipper8)
7) Flip phone (vertically) to go to phone mode, but you need to switch in on in the phone mode option first (alexei)
8) ctrl+backspace to bring up the task switcher (amh)
9) ctrl+shift+x to bring up a (new instance of) terminal (amh)
10) Put both thumbs of either side of the desktop and release 1 thumb for quick desktop transition (nibbles)
11) When in the web browser hit the back space key to go to the last page (jinlaca)
12) When in the browser, use the volume rocker switch to zoom in and out (rmerren)
13) Ctrl+Shift+P snapshots the current screen (Fargus)
14) Double tab the blue arrow to lock blue numbers and characters on the keypad. The same apply to the up arrow for all CAPS (x61)
15) flipping the phone from screen face up to face down snooze the alarm (hwong_pz)
16) From the contact list, pressing Ctrl + n allows you to add a new contact. Also, pressing the same buttons while on a web page brings up a fresh window (x61)
17) Open a terminal and type 'lshal | grep perc' to see exactly how much battery you have left (rdnetto)
18) Type 'lshal | grep batt' to see all the battery specs. (rdnetto) Alternatively, 'hal-device bme' (9000)
19) In the default browser, wipe screen clockwise/anticlockwise to zoom in/out (bunanson)
20) In video/audio/internet/radio player you can search just by typing just like in contacts. (slender)
21) In the camera application, the top rocker zooms in and out (Jerome)
22) It is well known that in the browser, the spacebar scrolls the page downwards, and the backspace key goes to the previous page in the history. It's a bit less well known that the same keys, shifted, have the opposite effect :-) (fpp)
23) A long press on the desktop brings up the Desktop menu straight away instead of having to press on the desktop then press the circle button (casper27)
24) If an application you are using such as the Browser freezes up and is unresponsive. Click the power button and choose End Current task. Just like in task manager in windows (casper27)
25) You can turn off the auto word completion/capitalisation in the text input settings (Dervish)
26) Using internet with the keyboard closed, you can swipe from the left of the screen to get cursor mode - but you can also swipe from the right of the screen to bring up browser history (JohnLF)
27) Once you've done the left-side swipe in browser, use finger to pull mouse cursor to point in page, then use [space bar] or [shift] to left-click and select area the cursor is over - useful for e.g. scrolling back to a point in a flash video (digital909)
28) You can enable turning to switch to portaint mode in browser's setting. (9000)
29) You're able to use the keyboard while browsing in portrait mode by closing the keyboard just a bit. Thus enabling you to browse with one hand by pushing space with your thumb. Comes handy if you're reading long texts (chainreaction)
30) Reflow the text in the browser when you are zoomed in with control+shift+i (mikec) (Doesn't seem to work for some people)
31) In mediaplayer "Now playing" tap the albumart to toggle between the current song and a list of songs on that album (m1cke)
32) Anyone know how to autocomplete a word when the keyboard is closed? yes. u tap on the orange part of the word as opposed to the end of it to complete it. if pressed at the end it'll cut it off. took a while to figure that out lol (badsubby)
33) You could install shortcutd and use the proximity sensor (TomJ)
34) In the web browser you can use ctrl + right arrow to go to the next field in a form (like hitting TAB on a normal keyboard). ctrl + left arrow acts like shift-TAB, going to the previous form field (whaleyboy)
35) Make a folder with a name starting from dot in MyDocs. eg .folder, then use the path file:///home/user/MyDocs/.folder to access the contents of this folder..the files here are not visible in any of the music n video players, but when u open them from browser they work just fine (aman18narang)
36) Creating a folder simply called 'private' will stop it being indexed in videos/images/music also (whaleyboy)
37) Open up the X Terminal, and type the following: apt-get moo (Venemo)
38) To move between the text fields on a form (like user id / password email settings etc) you can use just the up and the down arrows. These act as 'tab' and 'shift + tab' on a pc (navsin)
39) Press Tab in Xterminal for text autocompletion, e.g. cd M<press Tab> (scouser)
40) Ctrl+b bookmarks a page (x61)
41) Similar to typing a letter to jump to your contacts, if you enter a number it immediately brings up the phone. This is the quickest way to add a new contact (JohnLF)
42) Give the screen a long press. That will give you the context menu (Jesper Cheetah)
43) Double clicking the call button on a BT headsed re-calls the last dialed number (Bec)
44) If you're playing a movie with mplayer or SiB+mplayer on a TV with TV-out connected, you can swap out of movie in N900 by pressing CTRL-Backspace, but the TV is still playing the movie fullscreen! You can then do something else with your N900 like browsing webs or checking mails while at the same time watching movie on TV. You just can't do that with your default mediaplayer (9000)
45) Shift + up/down arrows go to top/bottom of current page (fpp)
46) Press Ctrl + Shift + R in any application. Then close the keyboard and turn the phone into portrait orientation. Most apps will auto-rotate then. (pycage) (Doesn't work for some device)
47) Install rootsh, type 'root' in xterminal to gain root access (9000)
48) In xterminal, gain root(47), enter 'reboot' reboot the device (atilla); or 'init 6' to reboot; 'init 0' to shutdown the device without restart (LippiVan)
49) You can print images if you send them to a Bluetooth capable printer via Petrovich (stickymick)
50) In PR1.2, with long-press any key, you can access to the symbols/numbers on those keys without having to press the blue arrow key first (JorgeFX/rickysio)
51) Choose 'Mass Storage' mode when you connect N900 to PC via USB for faster charging rate (9000)
52) Press the right-arrow key for text-autocompletion. (paulkoan)
53) In additional to point 46: hold CTRL+J+M+K+SPACE+L (L must be last) to turn the desktop(!) and current app into portrait mode (it works for firmware week 19 or after) (9000)

Source:
http://talk.maemo.org/showthread.php?t=54429