Segan, the routers don't really run a custom OS, it is VxWorks, which is ubiquitous in embedded devices.
That, and you can still get the linux one, but it is now labeled WRT54GL. Still works great. I personally have a WRT54GSv3 (8MB flash 32MB ram, gotta love it!).
As for what router will be the best, the first response was quite accurate. It depends on what you need. If you were more specific about how many users and how heavily they use the internet that would help a lot. With that being said, if you are a do it yourselfer, then running pfSense, m0n0wall, or smoothwall on an older PC is great. If you just want it to Bleeding Work(tm) then you should go for a WRT54GL, flash it with a good easy to use third party firmware such as DDWRT, config it once, and let it sit. For a home user, even a group of home users, or even a small lan party, anything more then a homebuilt or a WRT54GL-style (more brands sell linux routers now, the market took off!) is severe overkill.