The camera is the crucial component. Some factors to consider when purchasing...

1. If the camera and computer are within 15 feet they can be connected directly with FireWire (IEEE 1394). Active repeater cables can be used to extend this further. For longer distances an Ethernet or wireless link using Internet Protocol (IP) would be best.

2. For good sensitivity under low light conditions a CCD (Charge Coupled Device) sensor is required. CCD sensors also respond more rapidly to changes in light levels so are better for recording video. Most cheap webcams use a CMOS sensor which is less sensitive.

3. For sharp images auto-focusing would be of benefit. Fixed focus webcams intended for short range use are unlikely to provide quality images from 15 metres.

4. If the camera is to be mounted outside it will either need to be design for external use or housed in a weatherproof enclosure.

5. Cameras are available with built-in web servers. However, to facilitate recording and the triggering of alarms a host computer running webcam software may be preferable.

A good low-cost CCD FireWire camera is Apple's iSight:
http://www.apple.com/uk/isight/
>Quote: "Featuring an autofocusing autoexposure F/2.8 lens which captures high-quality pictures even in low lighting, iSight also includes a dual-element microphone in its stylish compact aluminium body."

Another is Orange Micro's iBOT FireWire Web Cam:
http://www.orangemicro.com/ibot.html

For some IP cameras see Broadband Buyer:
http://www.broadbandbuyer.co.uk/Shop/ShopSearch.asp?CategoryID= 98&ShopGroupID=32&Alt=Yes

and Transparent Communications Ltd:
http://www.transparent.uk.com/acatalog/IPCameras.html

Software
Two good programs are TinCam (Windows, $19 shareware, 30-day free trial):
http://www.tincam.com/
>Quote: "Monitor your home, your car, your office etc. with TinCam. When TinCam detects motion it can grab a picture and send it to you in an email if you're away from your computer."

and EvoCam (Mac OS X, $25 shareware, 15-day free trial):
http://www.evological.com/evocam.html
>Quote: "EvoCam 3.5 introduces support for network cameras and video servers, adds an access log and improves streaming performance for the built-in web server, offers enhanced motion sensors, and features an entirely rewritten user interface with live resizing and scrolling."

In addition to its web server, EvoCam also supports Zero Configuration networking:
http://www.apple.com/uk/macosx/features/bonjour/

For a list of suitable cameras see 'EvoCam Supported Cameras':
http://www.evological.com/cameras.html
>Quote: "In addition to any QuickTime-compatible camera or video input device, EvoCam can be used to view, record or re-broadcast video from network cameras. EvoCam can also control pan and tilt on some network camera models."

For a tutorial see 'HOW-TO: Turn your laptop into a home security system':
http://www.engadget.com/entry/1234000653032760/
>Quote: "With almost any generic webcam and some relatively cheap software, you can set up a motion-detecting security camera, a periodically refreshed image of your home interior/exterior, or even a live video feed direct from the excitement of your empty living room."

Also see 'Mini Media Mac: mini WebCam Security System':
http://www.macmerc.com/articles/Mini_Media_Mac/288
>Quote: "The mini's small footprint and low cost make it perfect for dedicated tasks. In this tutorial, we'll set up a wireless surveillance system."

As suggested in both those articles, Apple's Mac mini is a good choice for running Evocam -- it's cheap to buy and only consumes around 20 watts of power:
http://www.apple.com/uk/macmini/