Can anyone recommend a good real-time intraday scanner that will detect things such as stocks making new highs/lows, spikes in volume, large block trades, etc.?
Any discussion concerning real-time scanners that doesn't mention Neovest's First Alert would be lacking, IMHO. Arguably the most powerful real-time scanner to be found.
There are many scanners out there that will probably do the things you want (and a whole lot more). Like most software, some are more flexible and some use proprietary algorithms. A few you might want to check out are:
The last two are very similar, and if I'm not mistaken they were designed by the same programmer. With Realtick there's also Hottrends (built-in), and the OzScans can be added for a nominal fee.