I love my slow cooker! For beef casserole I do as follows:
Fry off the beef till brown, put it in slow cooker with loads of chunky veg - carrots, mushroom, shallots, leeks etc etc. Put in some mixed herbs and about 200ml of beef stock. Leave it to cook all day and about an hour before its done I stir in a little bit of gravy powder to thicken it up. DELISH!!!!
I also do chicken & tarragon casserole in much the same way except use chicken stock instead of beef, add some dried tarragon and towards the end put some chicken gravy granules in.
Chilli and spag bol always come out well in slow cooker too - just remember to add slightly less liquid than you normally would when cooking on the hob as the liquid doesn't evaporate like it does when cooking on a hob so you end up with lots more liquid than you'd have thought!