Just to weigh in on a delayed basis; I've got my rainy day fund in a building society saver account (negligible/negative interest rate), some in premium bonds (slightly less negligible/negative interest rate) but most of my extra money gets put in to overpayments on my mortgage.
My mortgage interest rate is relatively high at 3.2% (I was at the very limit of what they were prepared to lend me and I wanted a fixed term in case interest rates went sky high, even though the monthly repayments would have been less than the rent we were paying), but apart from that paying it off early is a double-whammy - not only do you end up forking over much less money to the bank in pure interest, but you'll also stop paying those mortgage payments much earlier and will thus start pocketing the money much sooner. You might be able to get technically better returns with stocks etc, but you're paying off debt on something that's literally safe as houses. Because of the magic of compound interest, the more money you can pay off sooner in the mortgage, the bigger the benefits, so most of my money designated for savings is now piled in to the mortgage instead. My mortgage with nationwide allows me to overpay by up to 10% per annum of the total amount borrowed before incurring penalty charges (and I've got the option to not pay my mortgage if I need to spend the money so it also acts as a safety buffer); this sort of arrangement seems fairly standard with most other people I know with mortgages (although of course the banks don't like to draw attention to it).
I didn't see it mentioned skimming the thread, but MSE have an overpayment calculator where you can chuck in your own circumstances and see if it makes financial sense for you:
Mortgage Overpayment Calculator shows how much you can save by paying off your mortgage early - if your mortgage allows overpayments.
www.moneysavingexpert.com
You need to factor renewed/re-mortgages as well, but even if you're optimistic and end up on a low interest rate for the entire term, the savings can still be substantial. For example, say you borrowed £200,000, paying back at 1.5% for 25 years; overpaying by an extra £2k a year means you pay £8k less interest and finish your mortgage 5 years early (netting you an extra £48k you would have otherwise been spending on mortgage payments) which is pretty impressive. Manage to overpay by £5k a year and now you're saving £15k in interest, finishing 9 years early and saving yourself £89k in mortgage payments.
I used to have both a stocks and shares ISA and some investment funds which did very well for me when I was saving up for a deposit and I'll likely resume these whenever the mortgage gets paid off, but for now my mortgage is my biggest earner.