This one might be a little too obvious - you need a breast pump and a power cord. (Especially if your pump doesn't have a battery.)
You'll obviously also need your pump parts. Pack them in a wet/dry bag or a zip-top plastic bag. Putting them together before you pack can help you make sure you don't forget a part.
A breast milk cooler can help you keep your milk cold on your commute home (or at work if you don't have a fridge there.)
If you don't have a wearable pump or hands-free cups, you'll want a hands-free pumping bra so you don't have to hold up your pump parts while you pump.
Other items that can be helpful: lactation massager, breast pads, nipple cream, breast pump wipes, bags.
Download a printable checklist here!