CandyLaser
Adventurer
A few other possibilities:
- The Dark Matter setting for Alternity (original recipe) and D20 Modern features your more conspiracy-laden, X-Files-esque alien invasion.
- Back in the distant days of AD&D 2e, there was a boxed set called Tale of the Comet featuring a 'standard' D&D world getting invaded by alien robots; there were also a couple of Dragon Magazine articles in the 3e days with a similar conceit. Take a look at Dragon #258 and #270 if you're interested.
- Torg is a cross-dimensional invasion, rather than traditional aliens, but is thematically similar.
- One game I don't own and haven't read is Arion Games' Invasion for Savage Worlds; it's on DTRPG, but I don't know anything about it beyond that it's about an alien invasion.
- GURPS has been mentioned. I'm not aware of a supplement specifically covering alien invasions; I think you'd have to roll your own using GURPS Space (for 4e) and GURPS Aliens (3e, and mostly compatible with 4e). There is also GURPS War Against the Chtorr, which fits the brief but only covers one specific fictional alien invasion.