MpegALot has one purpose - to facilitate the encoding of large amounts of videos to the MPEG4+AAC format (for ipod, PSP, etc). It has several useful features that greatly simplify this task (bolded items are ones I find especially useful and/or have not found in any other Batch MPEG4 encoders):
Clean, unified, streamlined GUI that handles all the various "behind-the-scenes" open-source command-line encoding apps
Encoding frontend is completely multithreaded to take advantage of multicore/multiprocessor/hyperthreaded systems to greatly speed up batch encodes
Accepts both files and folders as sources, and can apply custom filtering rules to specify only certain files/folders to encode
Can recurse through source folders to encode all files in all subdirectories
Output all encoded files to a single folder or to a directory relative to each source's path
Can preserve metadata from source to output files
Multi-user aware, so multiple users can run different encoding jobs without interfering with each other
AV presets and encoding jobs can be imported/exported as simple XML files for easy transfer Between machines
"AutoRun" mode suitable for launching from the command-line or task scheduler
Extensive logging mechanism and GUI for debugging errors and monitoring Progress
Able to encode almost any file that is playable in Windows Media Player
And so on...
