Zap alike game!

#1 Subspace (aka Continuum) is a venerable game, originally from Virgin Interactive, created as an internal test by their programmers to test Network/multiplayer/latency code. However, it is only superficially similar to Zap.

Zap is a lot more focused, with clearer goals, better controls, and simpler graphics. I also far prefer the way bullets and missiles behave in Zap.

Continuum is fully free, so I recommend trying it out, but it is not very friendly to noobies. Don't expect help from the people who have been playing it for 5 years and just see you as an easy snack. The easiest game to just hop in and play is... er... I forget the name. But it's the most popular server of all, regularly having 300+ players on it, and almost always with the largest population.

Myrddin

#2 Not to mention the gameplay is itself different.

Robotron, Smash TV...those games are similar to Zap.

#3 Zap could also be considered to be more similar to Tribes and Halo than Subspace.