One of the apps I've been keeping for a long time is
Sol Free Solitaire. It has 6 different types of Solitaire card games namely:-
1) Baker's Game
2) Baker's Game Easy
3) Demon
4) Klondike Deal 1(the one we all know and play when we're bored out of our skull)
5) Klondike Deal 3
6) Spiderette
The design is simple and nice. And aside from Klondike Deal 1, the rest range from quite challenging to very challenging.
And I find
The Quest Lite by Chillingo Ltd very, very engaging at the moment. It's the lite version that I'm playing and yet I've been playing for hours since yesterday.
Other cool ones that I've played again and again:-
1)
Who Has the Biggest Brain? (A series of tests to see how big your brain size is, ranging from amoeba to gorilla to genius, etc. Not to be taken too seriously but fun nevertheless)
1)
StoneLoops! of Jurassica (More than an arcade jewel-shooting game. You can upgrade your houses, has different layouts - deserts, glacier, jungle, etc)
2)
Golden Miner (Makes you feel happy picking up all those gold and diamonds

)