Any sort of MMORPG, as well as RPGs that have an active mod community, (or even FPS with the same) would be great timesinks.
The key is "new content". MMORPG often introduce new content periodically to get you to come back. RPGs and FPSs now have DLCs, and often, fan-made mods and such that rivals official DLCs in size and playability.
For example, Fallout New Vegas is a game that you can easily sink 100+ hours in the main game alone. There are four official DLCs which will add another 20+ hours easily. Furthermore, there are fan-made mods that added missions / quests / weapons / locations / companions / bug fixes / map areas and more. Some are just infantile (like "naked mods", but others are absolutely amazing missions with storyline, such as "New Vegas Bounties". And they add a LOT of replay value.
FPS, with multiplayer and some randomization plays differently every time, but often they have plenty of new map packs. MW3 is adding new map packs every month, BF3 has "Return to Karkand" DLC,