well i figured since i get quite a few pm's about me using blender for halo related projects that i would make a getting started thread for anyone who wants to try their hand at blender.
blender is an open source modeling program that has been rapidly improving over the years (if you had asked me 5 years ago about blender i would have said it was utter crap) it has most of the features of major programs all in a free to use package.
http://www.blender.org/ For complete newbies watch this video tutorial series: https://www.youtube.com/watch?v=lY6KPrc4uMw&index=1&list=PLda3VoSoc_TR7X7wfblBGiRz-bvhKpGkS some of those tutorials are also helpful if you are coming from 3ds max or other modeling programs.
when compared to other modeling programs blender seems a little lacking in gui buttons. it primarily uses hotkeys for most of your standard commands. (if you really feel like wasting a lot of time to you can change every single hotkey in user preferences)
if you know the hotkeys for 3ds max there are hotkey presets saved online for different programs such as this one
http://blenderartists.org/forum/showthread.php?186905-3dsmax-hotkey-preset-for-Blender one of the first things you will want to do when you install blender is go into preferences, in the input tab youll want to change these mouse settings on the left side bar
otherwise blender has this really weird trackball style view which is really disorenting to use.
the emulate numpad function in the input tab allows you to use the number keys to change views. i personally like to use it even if i do have a numpad.
the veiw keys are normally
1 for front view
2 rotates the camera down
3 is right view
4 rotates the camera left
5 switches between orthographic and perspective views
6 rotates camera right
7 is top view
8 rotates camera up
some people like to change those around but i personally like to leave them how they are.
here is also a nice cheat sheet for the standard hotkeys

as far as workflow into halo ce goes we still have to use 3ds max. but only for export. the latest version of the fbx exporter keeps the scale from blender to max correct as well as carrys all your uv data and geometry. make sure you use the triangulate faces function before exporting in order to save yourself a lot of trouble
once the map is in 3ds max check for flipped faces. sometimes you will get odd normals when importing your .fbx (not sure if it is 3ds max's fault or blender's)
then from then on it is the exact same export procedure as shown in the hek tutorial
there was at one time a blender exporter plugin that was being worked on by a user here. but it has been a long time since i have seen him around and im pretty sure work on it has been discontinued.
here is a link to that thread if you wish to take a look:
http://forum.halomaps.org/index.cfm%3Fpage=topic&topicID=42486