Game programming for the Commodore 64 may have slowed to a crawl since the 1990's, but musical remixes of the best tunes from C64 glory days are alive, thriving, and readily available thanks to sites like Remix.Kwed.Org.  

There are different categories of C64 remixes, ranging from lightly "inspired by" songs that use melodies from old C64 games but are otherwise nearly indistinguishable from any other modern music, to songs that fully integrate original SID chip sounds for a more retro feel. 

Chronberg (also listed some places as "Chronblom") is an arranger listed at RKO whose tunes fit into the former category, and I've been quite impressed by the results.  It's the kind of music you can blare out of your car windows without a geek alert sounding.  That's not to say that all SID-type music is geeky (most certainly not), but a couple of Chronberg's songs sound like something you might hear at a club and not be any the wiser, even if you're a C64 fan with an ear out for familiar tunes.  Neither of these two games are ones I had ever played back in the day, which is not surprising considering the huge number of C64 games produced over the years. 

I heartily recommend giving Mark Alphingwood's inspired (Mark being the original SID tune's creator) Gyroscope - Unzalicious version a try, and then Matthew Stevens' Geo-Matrix (Schizophonic version) .  That is, unless you just really don't like techno/trance style music.

After that, why not check out the games that inspired the music?

Geo-Matrix

Gyroscope