... MPEG Streamclip is a powerful free video converter, player, editor for Mac and Windows. It can play many movie files, not only MPEGs; it can convert MPEG files between muxed/demuxed formats for authoring; it can encode movies to many formats, including iPod; it can cut, trim and join movies. MPEG Streamclip can also download videos from YouTube and Google by entering the page URL....
Those are impressive features. The post claimed conversion quality equalled that of Apple's Compressor app.
Reading the release notes it's typical video software - somewhat flaky and tricky to use. Some functionality requires companion apps. It's not fully Lion compatible and OS updates may require app reinstallation. Some details on functionality (emphases mine)...
MPEG Streamclip lets you play and edit QuickTime, DV, AVI, MPEG-4, MPEG-1; MPEG-2 or VOB files or transport streams with MPEG, PCM, or AC3 audio (MPEG-2 playback component required); DivX (with DivX 6) and WMV (with Flip4Mac WMV Player). MPEG Streamclip can export all these formats to QuickTime, DV/DV50, AVI/DivX and MPEG-4 with high quality encoding and even uncompressed or HD video.
Video conversion is performed in the YUV color space; you can choose to scale video to any frame size using a professional 2D-FIR scaler (better than bicubic) but you can also leave it unscaled. Other optional video processing features include a powerful motion-adaptive deinterlacer, a field dominance converter, a chroma reinterlacer and an option to perform interlaced scaling instead of progressive scaling. Audio can be converted to uncompressed or to IMA, AAC, MP2 or AMR using the high-quality MP1/MP2/AC3/PCM built-in decoders of MPEG Streamclip; it is also kept in perfect sync with video using a timekeeping system.
MPEG Streamclip can save edited movies as MOV files, and (when possible) as AVI or MP4 files. Edited MPEG-1 or MPEG-2 files are saved as MPEG or TS files.
MPEG Streamclip (with or without the MPEG-2 Playback Component) can also convert MPEG-2 transport streams into muxed MPEG-2 files, for immediate burning at full quality with Toast 6 or 7 and Sizzle; it can also demux MPEG-1 and MPEG-2 files and transport streams with MPEG, AC3, PCM audio to M2V and AIFF (or M1A or AC3) files, for immediate burning at full quality with DVD Studio Pro or Toast 6 and 7. A special demuxing option is available for Final Cut Pro 4/5: this application does not work well with M2V files, but MPEG Streamclip can write a special "unscaled" M2V file that preserves full video quality when imported into Final Cut Pro.
MPEG Streamclip can handle files and streams larger than 4 GB, split in any number of segments, or with multiple audio tracks, and can also optionally handle timecode breaks. It is compatible with MPEG-1 and MPEG-2 video, MPEG layer 1/2 (MP1/MP2) audio, AC3/A52 audio, and PCM audio.
The player included in MPEG Streamclip lets you preview the files and transport streams before doing the conversion; it also lets you visually set the In and Out points for the conversion so you can convert just the part of the file you are interested in, and also cut commercials and other unwanted parts, or edit the stream and join two streams with Cut/Copy/Paste.
MPEG Streamclip supports batch processing: just drag some files in the batch list, choose a conversion and a folder, click the Go button and MPEG Streamclip will automatically convert all your files.
Reading between the lines I suspect it's sometimes used for non-approved editing of commercial products.