GSO's 1.25" 2x "Shorty" achromatic Barlow is an excellent accessory that allows you to effectively double the number of eyepieces in your collection. This barlow uses two-element, fully-multicoated lenses with excellent color correction and sharpness. The lens edges are blackened to maximize image contrast. The body is precision CNC-machined from aluminum and sports a handsome finish.
A really nice feature of this GSO barlow is that the barlow element is housed in a separate metal cell at the bottom that can be unscrewed from the upper tube (see photos). You can then screw the barlow cell directly into 1.25" eyepieces with a standard 1.25" filter thread to increase the magnification of that particular eyepiece while still maintaining a low profile. When screwed into an eyepiece, the barlow will yield approximately 1.5x of magnification. So, for example, a 30mm eyepiece can be used with the barlow element screwed on as a 20mm eyepiece, or with the complete barlow as a 15mm eyepiece. That's like having three eyepieces in one! You can also use the tube by itself without the barlow element as a 36mm (1.4") extension tube for 1.25" eyepieces and accessories.
Note that the use of any optical element like barlows and focal reducers changes the focus point of your telescope, requiring additional in-focus or out-focus travel. The exact amount of focus travel required will depend on the optics and mechanics of your specific set-up (telescope, diagonal (if used), barlow, and eyepiece). Hence, be aware that you may not be able to reach focus with all eyepieces or in all configurations with a given barlow.