Great deal via cheapstingybargains.com
Meritline has an Entry Alarm for Doors and Windows for $2 after $1 off coupon code MLC4010971 (expires 6/10/09), which is 33% off the $3 regular price. Shipping is free.
The Entry Alarm for Doors and Windows will set off a 90 dB alarm if the door or window to which it is attached is opened more than 1.5 cm. It attaches without tools, features a simple on/off switch and is small enough to be portable.
Great for doors, windows, keeping track or toddlers and teenagers who think the rules were made to be broken...