Although Steam and its ilk coax me into buying new games regularly, it's uncommon for me to beat them and it's even rarer that I take them for a second spin -- especially once I've abandoned them. However, for whatever reason, a handful of games still manage to hold my interest after five or more playthroughs.
I fire up Splinter Cell and Hitman several times a year and I've played through Chaos Theory half a dozen times since 2006ish. Collectively, I've beaten titles in those franchises more than 20 times. Similarly, on the multiplayer side, I've repeatedly revisited StarCraft (and now StarCraft II) for a month or so every year since 2002.
It seems I'm not alone here. Most gamers I've spoken with replay certain titles or series semi-regularly, whether for nostalgia or simply because they can't have the same experience elsewhere -- I believe the latter applies to my fondness of Splinter Cell and Hitman. Have you revisited any games several times, and if so, why?