Sparkfun

US$7.55 US$6.99
This is a small board that connects to the Raspberry Pi’s 26-pin GPIO connector, and breaks the pins out to breadboard-friendly arrangement and spacing, and even adds a couple of decoupling capacitors on the power supply lines.
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US$39.05
With the XBee Explorer Dongle you can plug the unit directly into your USB port and have it act as a gateway between your computer and the XBee. No cables needed! This unit works with all XBee modules including the Series 1 and Series 2.5, standard and Pro version. The on-board voltage regulator is good up to 500mA.
Delivery within 2-3 weeks
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US$2.03
This is a 2x23 pin male PTH header. This piece is in the common configuration for Beaglebone Black headers that can be broken apart to form smaller header formations. Each pin has a spacing of 0.1" in between each other.
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US$4.48
This is a 2x23 pin female PTH header, with extra long legs which makes great for stacking. This piece is in the common configuration for Beaglebone Black headers. Each pin has a spacing of 0.1" in between each other.
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US$26.61

If you need to charge LiPo batteries, this simple charger will do just that, and do it fast! The USB LiPo Charger is a basic charging circuit that allows you to charge 3.7V LiPo cells at a rate of 500mA or 100mA. It is designed to charge single-cell Li-Ion or Li-Polymer batteries.

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US$15.32
These cases protect your Arduino Uno from things like rogue wires that might short it out while still allowing full access to the board!
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US$24.20
This little breakout that uses the DS1307 to keep track of the current year, month, day as well as the current time. 
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US$9.73

This is a great inexpensive option for wireless communication; it's an ISM band FSK transceiver module implemented with a unique PLL. These modules operate in the 434MHZ band and fully comply with FCC and ETSI regulations.

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US$15.33

This is a small and very easy to use audio sensing board with three different outputs. The Sound Detector not only provides an audio output, but also a binary indication of the presence of sound, and an analog representation of its amplitude.

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US$0.96
This is a simple 2x3 male PTH header. This header is in the common configuration for an ISP connector. Each pin has a spacing of 0.1" in between each other.
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US$4.92
This is the Mini Solder-able Breadboard. A bare PCB that is the exact size as our mini modular breadboards with the same connections to pins and power rails. This board is especially useful for preserving a prototype or experiment you just created on a solderless breadboard by soldering all the pieces in place.
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US$67.50

This complete module is built upon the same technology as the ET-301, but includes on-board voltage regulation, LED status indicator, battery backed RAM, and a built-in patch antenna! 6-pin interface cable included. This EM-506 GPS module is powered by SiRF Star IV, it can provide you with superior sensitivity and performance even in urban canyon and dense foliage environment.

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US$7.14
This is a simple board that breaks out a female USB type A connector's VCC, GND, D- and D+ pins to a 0.1" pitch header. If you want to add a USB to your project, but don't want to put up with soldering tiny connectors, this is the board for you.
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US$2.73
These 6-pin connectors are very similar to the RJ11s we use here in North America except instead of a bottom-mounted hook lock, the BTA uses a side-mounted lock. These BTA connectors are perfect for educators or other users who need to set up a Custom Vernier analog sensor which produces a voltage that changes from high to low or vice versa.
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US$5.53
With 170 tie points there's just enough room to build and test simple circuits; They're also great for breaking out DIP package ICs to jumper wires! If you run out of room, no worries, these mini breadboards can be snapped together to form larger stretches of board. Dimensions: 47 x 35 x 10mm.
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