Sleek Logos is described as 'has hundreds of it industry logotypes simplified for icon size format. They come in various styles to match your design: grayscale, color, outline, or filled'. There are more than 50 alternatives to Sleek Logos, not only websites but also apps for a variety of platforms, including Windows, Self-Hosted solutions, Mac and Linux. The best alternative is Font Awesome, which is both free and Open Source. Other great sites and apps similar to Sleek Logos are Flaticon (Freemium), Iconfinder (Free), Fontello (Free, Open Source) and Icons8 (Freemium).
Futuramo Icons is a system of user interface icons with instant search and tools for size, color, style, swatches. The 16-style matrix makes the web app unique. It is a comprehensive, visually compatible, always expanding professional icon system.
iconmonstr - free, monstrously big and continuously growing source of simple icons run by one creator. Features: • Exclusive: Unique and uniform icons from an exclusive source. • Fill & Outline: Available in every release as fill and outline version.