From north east to south west:
There are a couple between Penmon Point (opposite Puffin Island) and Beaumaris, but the one that is in a fit state to be used is chained across with a notice saying private slip.
There's a decent concrete slip at Beaumaris (by the lifeboat station) but the bottom is sand. You need to be registered with Anglesey, Denbighshire, Gwynedd or Conwy and then pay the £10 launch fee to pier master. Parking on the village green.
Gallows point (near ABC Powermarine in Beaumaris) is a shingle beach and is free, although I've been warned that you need a 4x4. Plenty of space to park.
ABC Powermarine (www.abcpm.co.uk
) offer launching facilities and boatyard storage. Not sure about parking.
Menai Bridge slip, next to the bridge. Pay your £10 to Porth Daniel boatyard. Parking is a bit of a problem but the slip is concrete all the way down so is great at all tidal states. Need to be registered with one of the four councils above.
Y Felinheli (also known as Port Dinorwic) - there are several slips, but none that I'd really go for. They often get blocked by parked cars. There are apparently facilities at the marina, though I don't know any more about them.
Caernarfon - there is a concrete slip on the outside of the harbour wall at Victoria Dock. It does dry out at low tide and parking is quite limited although there is quite a bit of parking a short walk away on the pay and display near Morrisons. I think Menai Marine offer launching facilities as well.
I've also used Traeth Bychan, on the north coast of Anglesey, and cruised round (takes me about an hour to Beaumaris). Have to be registered as per Beaumaris and costs £10 to launch. There are toilets, cafe and plenty of parking (£1.50 per day), although it's crazily busy on a hot summer weekend. Slip is concrete but leads down onto sand. I've taken a Vauxhall Astra estate onto the sand with no problems.
That's a long post!
I've recently been on holiday to the area and investigated all the slips as the gale force winds kept me from actually launching.
Hope this all helps,