If you have just finished all the missions in standard EV:O and want to continue with your pilot, the best plug for you is probably Beyond the Crescent. It fits in very well with the established game culture, and doesn't add absurdly powerful ships or hardware which is likely to unbalance the game and spoil your achievement.
If you want a different slant on the game, with a fresh start but the same game universe, perhaps go for Reign of the Voinians and its sequel, and try joining the bad guys?
If you want to see EV:O, the best it can possibly be, download Frozen Heart, and its sequel, Femme Fatale.
If you want some no-nonsense little plugs that add a bit of colour, and don't upset the apple cart, look out for something like Mars Colony. (All it does is makes Mars so it's worth visiting.) Various other little plugs change the scanner images, the colour of the background stars, the shape of the asteroids... mix and match these little plugs in with whatever else you decide to use.
If you want some new gadgets to play with, why not download Dark Station? Be warned that some of the gadgets are a bit too powerful, though, and make the game too easy. The same could be said of (original) F-25 and The Fourth Reich. Just back up your pilot file, lob in the plug and enjoy... then remove the plug and go back to the old version of the pilot when you get tired of slaying everything in seconds.
If you want to play standard EV:O with different sounds and graphics, obtain a copy of Magma... preferably from a friend if you pay for your internet connection as the download time for some of the more elaborate plugs can be long. (Almost all plugs can be downloaded direct from Ambrosia, and most are mirrored on their creators' own sites.)
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(url="http://"http://www.geocities.com/evodude2001/home.html")Old & Unimproved: Cerberus Station!(/url)