I started play-by-post RP on Gaia Online as well and I moved entirely to Invisionfree and JCINK forums when I found them in 2004 (I was searching Livejournal RP ads, funny enough). The resources I used back then are basically the ones I use now:
RPG Directory has a system-agnostic directory that lists forum RPs. Since it’s been a while, I recommend checking out RP Request. There, you can say what you’re interested in and folks can drop their ads and give you the run down on what their site’s about and why you, specifically, should check them out.
RPG Initiative is one I haven’t used as much (mostly because I haven’t been looking), but attempts to give a little more information in their directory.
Barbermonger is the only forum I know of that is specifically for finding 1x1 RPs. I’ve used it with mixed success (RPers are notoriously flakey).
In addition, there are several directories on tumblr such RPG Adverts that list ads.
There are way more, but if you’re looking for forum RP, then those resources can easily get you to other places since RP sites normally host advertisement sections that guests can drop their ads on and they have affiliate buttons that links to major resource sites as well as other RP sites. If you’re willing to site hop, then you can find some interesting stuff by going through those advertisement sections.
That said, knowing what your preferences are would help! My experience thus far has been that the RP community is segmented according to seemingly unimportant criteria and while there’s crossover in the subcommunities on the above resources, some genres and preferences congregate largely in different places.