I've been looking for a tool to experiment with bitcoin script for a while now, and I finally finished developing one myself: https://bitauth.com/ide/

BitAuth IDE is an open source project for designing advanced scripts for bitcoin and bitcoin-like cryptocurrencies. It’s both a learning sandbox and a tool for designing new kinds of wallets.

It includes a live-updating, interactive, line-by-line debugger, which visualizes that state of the stack over the entire execution of the program. There's also an entity and variable system for automatically generating private keys, public keys, and correct signatures across locking and unlocking scripts.

See here for a more detailed tour and a video walkthrough:
https://blog.bitjson.com/bitauth-ide-write-and-debug-custom-bitcoin-scripts-aad51f6e3f44

And the source is on GitHub: https://github.com/bitauth/bitauth-ide