If you'll be riding after twilight lights are essential. Even if you can see just fine a light makes it easier for drivers to see you. A cheap hi-vis vest wouldn't hurt, again just to make sure you're seen.

If you're going to have to leave the bike out of your sight a lock is essential. Don't cheap out on the lock. My LBS told me of one of their customers who bought a top-end mountain bike and then spent the equivalent of about $20 on a lock for it (against their advice). Not surprisingly within a couple of weeks he was back - his bike had been stolen.

If you can get to and from work using some other mode of transport (be it train, bus or walking, or a spouse who can come and get you in the car) you can get away without things like a spare tube or puncture repair kit. Obviously you don't want to be walking 8 miles home in the dark and rain if you get a flat and can't fix it. Replacing a tube is easy enough to do but like others have said learn to do it before you've got a flat in the dark and rain.

Multi-tools are great but only if you know what to do with them. There's no benefit at all in having a bunch of assorted screwdrivers and wrenches if you break something in a remote area and don't know what to do with your collection of wrenches.

If you're not carrying a lot of stuff you can probably get away without panniers. A backpack isn't a lot of fun - it will makes your back wet through with sweat. If you're not carrying much you might get away with a small bag or a saddle bag. For slightly larger loads something like Topeak's DynaPack might work - it looks almost like a bubble that attaches to the seat post. It looks neat in principle although I've never used one so can't say if they are any good in practise. If you're routinely carrying lunch, laptop, change of clothes, washing gear including towel for showering etc then obviously you'll need something bigger.

Personally I wouldn't cycle without a helmet, but others would disagree on that.