Appreciate the answer has already been chosen, but thought it worth adding:
As of 2016/17 tax year, there is a new Personal Savings Allowance (PSA) in the UK which means you can earn up to £1,000 of savings interest, tax-free, every year without needing to use an ISA.
The level of PSA depends on your income tax band, as follows:
Tax band | Adjusted net income | Personal Savings Allowance
----------------|---------------------|-------------------------------------
Basic 20% | Up to £43,000 | Up to £1,000 savings income tax-free
Higher 40% | £43,001 - £150,000 | Up to £500 savings income tax-free
Additional 45% | Over £150,000 | £0 (no PSA)
As a result, all savings accounts now pay you your interest gross – they don't deduct income tax at source. (They simply tell the Inland Revenue how much they've paid to whom, so that your tax code can be changed if you end up owing income tax on interest in excess of your PSA).
So:
It's no longer generally true that a cash ISA will generally beat other savings accounts because of income tax on the interest
It also no longer seems to be the case that Cash ISA accounts pay inferior interest rates to other savings accounts. I just checked with 3 providers and the interest rates on offer were very similar.
You still might be better off choosing the ISA as it's a permanent tax shelter that accumulates every year. (i.e. if one day you get to the point that you're earning >£5k interest – on, say, £150k savings – you won't be able to move that lump sum into an ISA in one go). Also as Nigel Harper points out in comments, you can subscribe to an ISA while also earning your PSA worth of interest on savings outside an ISA as well.