After trails.com took over topomaps.com about five years ago, I had the same problem: no more easy to access free topographic maps online. That's pretty much changed now though.
You can find some great historic maps with all the trails on them at the Yosemite Library:
http://www.yosemite.ca.us/library/maps/
Also, here's a great tool to access topographic maps:
http://www.usgsquads.com/index.php/map- ... /mapfinder
I realize it's not online, but Jeffery Schaffer has a good 15 minute topographic map in his book,
Yosemite National Park. It features all the trails in the park. Schaffer also offers trail descriptions in his book, which I find very useful for planning purposes. I recommend it highly.
Of course, standard 7.5 minute USGS topographic maps are great and offer a bit more detail than Schaffer's map. I like to use National Geographic's TOPO! software, which I admit is not the most user friendly, to access them. TOPO! lets you draw your route, plot points, etc. so that you can gauge the length of your hike and get a feel for elevation changes. That said, generally speaking topographic maps have not been revised in a long time so they can reflect inaccurate trail configurations (but typically not far off in Yosemite).
Hope this helps.