Thank you all for your responses. While I certainly don't wish this behaviour on others, it's definitely a comfort knowing it's a somewhat normal stage and we're not the only ones facing the challenge.

I think it might help to consider any underlying factors here. Outbursts like this are often an expression of some unmet need. And little kids are often not too eloquent about letting you know when they need stuff.
You say he's frustrated about stuff - so maybe think about what you can do to help him with that before he loses his temper? (For my son - boredom and lack of attention from me are common ones.) Finding other 'physical outlets' for the anger can also help some kids.
More often than not we know exactly what the reason is for an outburst. It can be a simple as a toy not doing quite what he wants it to do, or us telling him he can't have something (like today he wanted a second tub of yoghurt and I told him he could have one after lunch, which was not right now so not good enough for him). After eating four slices of toast, a bowl of cereal and a tub of yoghurt I knew he wasn't still hungry, lol. Talking to him about it didn't avoid the tanty unfortunately.
I've tried introducing an alternative to the current frustrated behaviour. For example, I've tried to convince him to stamp his feet instead, and I even got a bottle that is filled with coloured rice and told him when he's angry he can shake the bottle really hard to make himself feel better and more calm. Hmm, maybe I should continue with that in conjunction with the time outs as I sort of gave up quickly with giving him alternatives.

And I guess many of you are right when you talk about them being worse when they haven't had enough stimulation. I'm sure I must be guilty of that at the moment. With probably only four weeks to go until the baby arrives I've been much less active with him, and now that DD is running around she takes more time and energy as well. Add to that our playgroup fizzled out months ago and we haven't found another one, and even our Mother's Group has missed a few sessions, so we've been rather housebound lately. I've just enrolled him in a Montessori toddler program that he should be able to start in the next three weeks. It's two hours once a week and it'll be the first time he's been away from me (besides when I had DD and the 3-4 times my parents have babysat for a couple of hours) so it'll be interesting to see how he goes. I hope he loves it. I think he'll be fine and I'll be the one in the car a blubbering mess because of MY separation anxiety, lol!