Tribes 1 and Tribes 2 were both released as freeware as part of the hype leading up to the release of Tribes Vengeance. You can grab either or both at FilePlanet.
I really don't know how many public servers still exist for the two games, but Tribes 1 was extremely 56K friendly (I was playing on a 24.4K connect back in the day and did quite well.)
In the field of web based MUD-style games, Kingdom of Loathing kept me entertained for a few weeks.
www.kingdomofloathing.com