Nice thread. 
Here’s a list of tools I have found interesting. Most of them are commercial software, but at least most of them have free trial/demo/limited versions available.
Trees and Plants
SpeedTree - http://www.speedtree.com/ (really high end - both quality and prize - even getting the trial requires a lot of paperwork)
XFrog - http://www.xfrog.com/ (comercial, somewhat dated - especially the stand-alone editor and “tune” utility)
ngPlant - http://ngplant.sourceforge.net/ (open source with liberal license for use of its libraries - editor is somewhat similar to Xfrog, but with less features)
There’s some other toolkits, but most of those are for very specific uses (and most of them very outdated). Most 3D software packages do have some sort of 3D tree/plant generation features, either built-in or available through 3rd party plug-ins. The most advanced I’ve seen is Vue (definitely worth a look for anybody interested in procedural stuff). Exporting models from these 3D packages might not be simple, since the programs/plugins often support dynamic level of detail.
There’s some decent open source libraries available if you search for “L-system” or “groimp”, but these seem to mostly be aimed at academia. I guess the main reason is that although these grammar based methods are very elegant, they are close to impossible for humans to edit by hand.
Textures
Filter Forge - http://www.filterforge.com/ (Awesome software, reasonably prized with trial available. Paying customers has access to their huge online library of textures that can be downloaded and modified free of charge. Definitely worth it to check out their online texture library on their website)
Genetica - http://www.spiralgraphics.biz/genetica.htm (A bit pricey - limited trial available. Has some unique advanced features, such as hierarchical graphs and animation support, but in my opinion not as easy and flexible to use as Filter Forge)
Darktree - http://www.darksim.com/ (Reasonably prized, trial available. Not as intuitive to use as Filter Forge, but does have some unique features including build-in rendering and a free plug-in API)
Terrain
There’s a lot of procedural terrain tools available, so I won’t try to list them all - I’ll stick to mentioning the most prominent (and ignore all the ones that aren’t node-based).
World Machine - http://www.world-machine.com/ (Reasonably prized, Free limited version available. Defintely my favorite terrain generation tool - it’s extremely powerful yet very simple and easy to use. Its fast preview functionality makes trying out ideas fast, easy and fun. There’s a plugin API available, but it’s not up-to-date with the newest version)
Terragen 2 - http://www.planetside.co.uk/ (Somewhat reasonably priced. Free limited non-commercial version available. Includes built-in support for Xfrog trees/plants including a somewhat pricey bundle of both Terragen and a big library of Xfrog plants. In my opinion Terragon 2 is over-hyped and not all that useful, unless you need the built-in render pipeline)
Mojo World - http://www.pandromeda.com/products/ (Somewhat reasonably prized. Free limited demos are available. This program has been around for ages and is supposedly quite advanced. However the user-interface is extremely weird and non-standard and my impression is that the learning curve is extremely steep. It also lacks basic features such as a live preview when editing node-graphs. From what I’ve seen it’s useless unless you really like creating alien-looking artsy worlds, but since I’ve never really managed to get anywhere with it I may not be the right person to judge it)
Cities
City Engine - http://www.procedural.com/ (Expensive! - even for the limited indie license. 30 day trial available. Looks very impressive.)
Suicidator City Generator (for Blender) - http://arnaud.ile.nc/sce/ (Free. Don’t have any experience with this)
Characters
Someone else already mentioned Mixamo, Morpheme and Make Human.
If you’re looking for inspiration Spore’s creature editor is worth a look too.
General Procedural Modeling
Houdini looks cool from what I’ve seen on Youtube, although it seems the recent versions focus less on procedural modelling and more on dynamic effects generation.
Generative Modeling Language - http://www.generative-modeling.org/ - might also be worth a look.
Google Sketchup is not truly a procedural editor, but it does come pretty close if you take the scripting functionality into account.
Procedural Sound
Not my strong area of expertise, but there’s some tools out there aimed at musicians (an old-school synthisizer is a good example of procedurally generated sound created in hardware). One modern software toolkit is Reaktor from Native Instruments: http://www.native-instruments.com/#/en/products/producer/reaktor-55/ (there are others, but I forget their names)
Would be cool if we could try and keep this thread as complete as possible. 