There are a number of different tools that can be used to develop for the MSP430. By far the most common are Code Composer Studio (CCS) from Texas Instruments and IAR Embedded Workbench from IAR Systems. Both are commercial products but both offer free code-size limited trial versions. However, the limits are greater than the RAM on many of the smaller chips, so they are essentially full-featured for many chips and many users. The current versions of both are Windows only but run just fine in a virtual machine under OS X, Linux, and likely any other operating system.
There are a number of open source development options as well. The most common is MSPGCC, a fork of the Gnu Compiler Collection. With varying levels of difficulty, MSPGCC can be compiled on just about anything.
It is important to note that Texas Instruments does not support OS X in any way, shape or form. In fact, they have publicly stated that they have no intention to support MSP430 development in the foreseeable future. If you are an OS X user, I urge you to contact TI and express your interest in OS X support. Because I am an OS X user and because I have had such difficulty getting started, I have dedicated a separate top level section to developing for the MSP430 under OS X.
CCS is based upon the popular Java-based Eclipse IDE. It is a bit of a resource pig, but in no way requires the recommended 4 GB or RAM. In fact, smaller projects can be built just fine in less than the 1GB recommended minimum. CCS sports a full debugger and the trial version allows for up to 16 KB of code. The current version of CCS (version 4) only supports Windows. The next version (CCS5) will support Linux as well as Windows, but no OS/X support is planned.
IAR is a much smaller, leaner, and (IMO) easier to use package. The trial version allows for up to 4 KB of code. IAR is strictly Windows only.
This is an native OS X IDE. No code-limited versions are available, but they do offer a 30 day evaluation license. Although OS X is my native OS, I have no experience with this product.
A port of the GNU C Compiler targeting the MSP430. MSPGCC is command-line based but can be integrated into IDEs like Eclipse. There are several different versions, but MSPGCC 4 is recommended in most cases.
The generic open source release of Eclipse can also be used with mspgcc and gdb back-ends.
Some other open source projects for the MSP430 can be found here.