Commercial FB projects are particularly interesting (I'd like to hear more), but you can also ask what are the largest FB programs? Well, I'm sadly ignorant, so I went looking for some. I'm going to count lines of FB code (including header files but excluding bindings and thirdparty libraries) because while we all know that's problematic, it does give an idea of the scale of the project.
I'm amazed how many GUI frameworks, GUI builders, IDEs, and libraries there are. Where are the significant applications and games (well there are numerous games, but which are major projects)?
FreeBasic:
125118 LoC of FB (the compiler itself)
plus 61195 LoC of C (rtlib & fbgfx)
plus 125053 LoC of FB (tests, including fbcunit)
plus 73049 LoC of FB (examples, many are translated from other languages, eg. NeHe)
plus 31755 LoC of FB (other utilities, mostly for doc generation, such as libfbdoc)
(Not going to include inc/. 503kLoC if you're wondering)
In total, 355570 LoC of FB
OHRRPGCE - essentially a recreation of a nineties dos game creation engine, to paraphrase marcov :)
151090 LoC (plus approx 30643 LoC in other languages and 13690 LoC in scripts)
fb-ext-lib FreeBASIC-Extended-Library
61719 LoC
WinFBE FreeBASIC Editor for Windows
45114 LoC (excludes examples, which seem to be almost entirely from WinFBX)
WinFBX Windows Framework for FreeBASIC
16009 LoC
plus 59289 LoC of examples (many ported from elsewhere, eg NeHe)
MyFbFramework - a forms building, drawing and etc. library for the FreeBASIC programming language
35323 LoC
fbfrog C-to-FB header translator
33702 LoC
fbrtlib (FB's rtlib ported from C to FB)
31075 LoC
VisualFBEditor: cross-platform IDE for FreeBasic with visual designer, debugger, project support and etc, based on the library MyFbFramework
30635 LoC (includes a few examples, there aren't many)
Prompt Critical a unique 3D first person perspective space shooter
23209 LoC
Station922 - HTTP server written in FreeBASIC for Windows
18190 LoC
libpruio Fast and easy Input/Output driver for digital/analog lines on Beagleboard hardware
9731 LoC (plus 4707 LoC in other languages)
FROST: 32-bit operating system based on a microkernel written in FreeBASIC
8179 LoC (plus a little asm)
(I'll edit this post if anyone suggests any notable large projects)