If you work in Windows choose thishttp://www.systronix.com/RAD51/RAD51.htm
RAD51 is an acronym for Rapid Application Development for 8051s.
RAD51 is an Integrated Development Environment (IDE) with a comfortable text editor, and an 8051 cross-assembler. It's a true 32-bit Windows application and requires Windows 9X or NT. Rather than tell you more here, you can check the data sheet or just download the whole program. It's free!
RAD51 takes a 'project' view of your code and retains basic information in a PRJ file. Shared code such as I/O routines can be kept in a 'parent' folder. Individual projects are stored in 'child' folders which can #include the shared code. We include several samples which illustrate this.
A $299 value (this lets you know it's worth a lot and not some cheap give-away), RAD51 is FREE!