Anyone who has ever smashed a thumb while trying to start a small brad or nail in a tight corner will welcome a handy new gadget called, appropriately enough, the Molly Thumb Saver.

Made by Bostik Consumer Division, USM Corp., Reading, Pa. 19605, and priced at $2.18, the device has a spring-loaded sliding jaw that is pulled back with one finger. When a nail is placed in the opened jaw -- it will hold anything from a 1/2-inch brad to a 10-penny nail -- and the slide is released, the nail is held firmly so the user can strike the head of the nail easily without getting the fingers near it.

The Thumb Saver also is handy for other jobs around the home and in the workshop: holding staples; holding wire solder when working in tight corners with small pieces; holding small pieces when assembling models or doing craft work. Electric Circuit Tester

A moderately priced Snapit Continuity Tester recently was introduced by Cable Electric Products, Inc., P.O. Box 6767, Providence, R.I. 02940. It differs from most previous models in that it has a bright indicator light right above the tip of the test probe where it can be easily seen when the tester is in use.

The Continuity Tester (costs about $2.49) requires two AA size batteries (not included), comes with a 36-inch lead wire that is connected to an insulated alligator test clip. The pointed probe or tip is protected against damage by a sliding plastic cover.

The continuity of any electrical circuit or wire can be tested by touching the probe to one side or end of the circuit being checked, while clipping the test lead to the other end. If the light glows the circuit is complete; if it doesn't, there is a break in the wiring. Tangle-Free Extention Cord

A heavy-duty electric extension cord is often needed when using power tools, lights or other appliances at some distance from the nearest electrical outlet. The problem has always been wrapping the cords for convenient storage without knotting and tangling.

The Caricord provides a neat answer to this problem. Made by Carico, Inc., P.O. Box 164, Somerville, Mass. 02143, it consists of a tough plastic carrying handle that serves as a sort of flat spool around which the wire can be wrapped when not in use.

One end of the unit has a molded handle for carrying or for hanging on a wall, and the other end has a three built-in, grounded outlet. The Caricor permits plugging in up to three tools or appliances at once without having to repeatedly change plugs. It also permits unwrapping only as much wire as is needed each time, while keeping the rest neatly coiled. Two models are available: The CR-25 has a 25-foot cord and costs $20; the CR-50 has a 50-foot cord and costs $29. Compact Tool Set

Small enough to be carried in a pocket or toolbox, a new 10-piece set of screwdrivers and nut drivers recently introduced comes packed in a neat, clear plastic case that snaps open to expose all the tools for quick use.

Called the Torque Commander Tool Set No. 70110, and costing $17.49, this handy tool kit is manufactured by Vaco Products Co., 1510 Skokie Blvd., Northbrook, Ill. 60062. The set includes the five most popular sizes of nut drivers -- from 3/16th inch to 3/8 inch -- plus two sizes of Phillips screwdrivers and three regular flat-blade screwdrivers and three regular flat-blade screwdrivers (1/8 inch, 3/16th inch, and 1/4 inch). Each tool has a short, smaller-than-normal handle, but a separate driver handle is included that fits all the others to greatly increase the amount or torque that can be supplied. Wallpaper Remover

One job most do-it-yourselfers do not look forward to is removing old wallpaper. It is usually a messy, time-consuming job that involves lots of scraping, soaking and-or steaming.

However, a new type of special release coating designed to be applied to the wall or ceiling before the paper is put up is now available to make any paper easily strippable.

Called Peelz Wallpaper Release Coating, and manufactured by William Zinsser & Co., 39 Belmont Dr., Somerset, N.J. 0,,73, it is a water-base resin coating that can be applied by brush, roller or spray. It dries in about one hour and acts as a sealer so that when wallpaper is hung over it (with the paste recommended for that material), the paper can be easily peeled or stripped off the wall at any time in the future.

The cost is about $4.25 a quart, or $12.50 a gallon, and it will cover about 700 to 800 square feet per gallon. It works with ordinary wallpaper as well as with paper-backed vinyls and similar products. Do-It-Yourself Storm Window

The Weather Window is a kit that includes everything needed to quickly and easily put up an effective, semipermanent plastic storm window on the inside of any existing window.

It comes in four sizes for windows and a special size that will cover patio doors up to 72 by 82 inches.

The kit includes a self-sticking white plastic channel that remains in place permanently. A clear vinyl sheet, included in the kit, is stretched over this and held in place inside the channel by a special plastic retainer strip that is pressed into place over the vinyl to grip it firmly.

At the end of winter or when the window is to be opened, the retainer strip is pulled out of its channel and the vinyl lifts off. It can be rolled up and reused next year by securing it over the channel with the same retainer strip.

Made by Tyz-All Plastics, Inc., 240 Glen Head Rd., Glen Head, N.Y. 11545, Weather Window prices vary from $4.99 for the utility size (up to 24 by 40 inches), to $8.99 for the large size (up to 50 by 80 inches). The super size for patio doors up to 72 by 82 inches costs $13.99. CAPTION: This device, called a Molly Thumb Saver, holds small nails or brads without endangering fingers. It costs $2.18.