Here's a list of what I am aware:

1) Maxis Dialup
2) TIME Dialup
3) DiGi Dialup
4) Celcom Dialup
5) TIME Webbit (Wireless Broadband using Navini technology, modem shaped like a rabbit)
6) iZZinet (wireless broadband)
7) Jaring Flite Wireless Broadband (terminated due to MCMC revokes the 2.5GHz spectrum allocated to Jaring)
8) AMAX (WiMax provider, launched services covering Damansara & TTDI, no expansion & eventually closed down)

Feel free to add any others that you know.

I won't be surprised if this really happen, it's only a matter of time. P1 since last year even offers Fibre/VDSL2 fixed line broadband, squatting on TM's infrastructure (just like Maxis Fibre) but I hardly know anyone using it.

But klseet had a bad experience & terminates it. You can read them here:

Everything seems not well since Michael Lai left. I terminated by P1 WiMax 2 years ago. I hope I still have the termination form with me, just in case I get a surprise letter saying I owe them 2 years of subscription fee.

I don't want to speculate anything but I read somewhere that he could not make his own decisions to bring the company forward.