Sound.jp is a web host of sorts and I found a nifty user page on it with tons of links to denpa songs. Most game companies (and sometimes anime studios) will offer its users a short version MP3 of the opening theme. This probably is an artist demand as well, given the short market penetration rate that these games sometimes have. Admittedly the quality varies from good to downright terrible - that's why this blog focuses on top artists such as KOTOKO and MOSAIC.WAV. So consider yourself warned and never forget that denpa and eroge are usually closely associated, so the NSFW warning is always in full effect.
About the page itself, once you scroll past the couple of adverts, you will come to two alphabetically-sorted lists. Clicking on a song title will bring you to an information box, with links to the game maker or sometimes even youtube. What I find hilarious is that the author included separate 電波 (denpa) and 萌え (moé) ratings for each and every song.
The link: http://sound.jp/dempa_song/