It's not all that sophisticated, but I'm using a text editor to write my BASIC program (TextWrangler on a Mac but could use anything really), then using the very handy "zmakebas" by Russell Marks which creates a .tap file containing a BASIC file, which I can then test into my emulator.  I find it a lot quicker than using the in built editor on the speccy (although you obviously miss out on the syntax checking as you go along, which is probably helpful if you're less fluent in Sinclair Basic).

I ended up doing something similar once I realized that BASin can load a hand-written .bas file just as easily as one saved by itself. BASin can only save snapshots, but that's good enough for my purposes.

If you want to create TAP files then BASinC (a fork of the original BASin with a few extra features and fixes) has an 'export to TAP' option which works really nicely.

Edit:

Oops, forgot the link. Silly me.

http://arda.kisafilm.org/blog/...

Doing the exact same thing. I'm also doing smaller experiments direct into ZXSPin with help from the SE Basic Rom feature, which lets you type everything directly. Its funny when you make a mistake in the code that the compiler doesn't catch and try to run it and get the error "Nonsense in Basic". ZX Basic doesn't pull any punches!   

Seems to me thats a pain in the ass when you want to test your code :)

I only curse BasinC when i want to move blocks of code arround...

I used notepad++ on win and the command line tool bas2tap to create the tap. How I miss a debugger! :) Of course some quick test on the emulator (fuse) before coding.

For the title screen, Paint.net, imagetozxspec to convert to scr and command line tool tapmaker for the tap.

Then good old copy /b command to put pieces together!

Last thing I would like to do is put it on an audio cassette an play it on my speccy, for real authentic experience :D