Avocet is a well-respected name in embedded development tools. They have two families of C compilers. AVCASE is $1595 and includes Avocet C, a macro assembler, a monitor debugger, and a simulator. Demos are available for download.
An ANSI standard C compiler that generates code for the 8051 family of microcontrollers. It provides numerous extensions that allow access to 8051 specific features so that you an write your code completely in the C language without the need to resort to assembler code. It supports both small and large memory models so you can create code for all 8051 variants whether or not they have external RAM. Includes compiler, libraries, assembler, linker, make, library manager, and IDE.
- Dunfield Development Systems
This reasonably-priced ($99.95) compiler includes an assembler, linker, librarian, library (with ASM source), monitor (with source), and various utilities and sample programs. Dunfield also has low-cost compilers available for many other microcontrollers (HC08, HC11, HC12, HC16, etc.). A simulator/debugger called "Emily" is also available for $49.95.
Franklin offer 8051 and 8051XA development tools including a C compiler, assembler, linker, debugger/simulator, RTOS, monitor, and emulator. Pricing ranges from $1395 to $1995 for the software tools.
A high-end compiler at a mid-range price (~$850), it boasts full ANSI C compliance, an optimizing compiler, assembler, linker, library with full source, and a source-level debug monitor. I briefly used this product for an 8051 project, and was very impressed with the code quality. Demos are available here.
- IAR Systems
I haven't used their 8051 tools, but their HC11 compiler is first rate. Pricing is competitive with other high-end compilers ($1395).
Keil's tools are well-regarded, with a full product line, including an assembler, compiler, debugger, and RTOS. They must be ashamed of their pricing, because they don't list it on their web site -- but here it is anyway: it ranges from $1395 for the basic compiler to $2995 for the "Pro Developers Kit". Keil's compiler used to be distributed by Franklin, but is now carried exclusively by Keil. A demo is available, or request a free CD-ROM.
Raisonance designs and sells a variety of development tools, including compilers, simulators, debuggers, RTOS's, IDEs, and emulators for 8051, XA, ST6, and Philips smart card products. €1095 to €2495. Free demos.
Rigel's READS51 combines an IDE, a SmallC-compatible 8051 compiler, an absolute assembler, a relative assembler, a linker/locator, an editor, and a chip simulator in one easy to use IDE. Freeware!
SDCC is a FREE, retargettable, optimizing ANSI - C compiler for the Intel 8051. The compiler source code is distributed under GPL. Available for both Windows and Linux.
Tasking has a nice-looking compiler for Windows. An evaluation version is available for download.
Unless stated otherwise, I have no firsthand knowledge of these products -- I'm just providing links. Where I do have some experience with a product, I'll venture an opinion. You're advised to take what I say with a healthy dose of skepticism -- my opinion is quite possibly out of date (based on old versions) or just plain wrong. On the other hand, I've been known to be right on occasion ... your mileage may vary. Also, I'm providing pricing because some vendors insist on hiding that critical piece of information (ashamed?); pricing was accurate when this was written, but will inevitably become obsolete as time passes. Please check with the vendor for current pricing.