A. Finance
Prof. Jayanth R. Varma’s Financial Markets Blog: Prof Jayant Verma is a Professor of Finance (mainly Financial markets) at IIMA. He is one of the most impressive teachers I have ever met. He was one of the first members of SEBI and is on the board of multiple big companies including Infosys and Axis Bank. His blog, which has been up since 2005, is a brilliant resource for understanding the nuances of the financial markets.
Dealbook (NYT): Dealbook is the news aggregator section of NYTimes to keep you updated on the current state of affairs of the Fin industry.
Mergers & Inquisitions: Everything you need to know about every aspect of the Financial world – from big investment banks to boutique firms, from Private Equity to Hedge Funds and AM/WM firms.
Investment Banking Blog: The holy mecca for the preparation of i-Banking interviews, the IB Blog offers everything from bootcamps and interview prep courses (paid) to free resources like basic interview questions, overview of the industry etc.
Wall Street Oasis: Similar to #2 above, but with more functionality to read the best articles of the month/week/day etc.
FT Alphaville: Another news aggregator that will make sure you do not have any time left in your day (courtesy it’s exhaustive coverage of current affairs)

B. Entrepreneurship

Entrepreneur: Your companion on the entrepreneurial journey. The site will keep you updated with the start-up world, help you work on various aspects of your professional self, and also provide the much-needed pep-talk when you are feeling down/exhausted/defeated.
AVC: The chronicles of Fred Wilson ( a Venture Capitalist for the past 30 years) with some incredible credentials to his name (including investments in Twitter, Tumble, Zynga, Foursquare etc). AVC is probably the most widely-read VC blog in the world.

C. Marketing

Mad Over Marketing: MoM is a fun blog. They capture everything from brand wars on twitter, to a collection of some of the best and most important marketing campaigns. The blog is a very easy read and will help you broaden your horizon when it comes to thinking of how you can ante up your marketing/advertising game.
Duct Tape Marketing: Duct Tape Marketing focuses on small businesses and startups. They take into account that small businesses do not have the funds that large corporations do. So their marketing tactics need to be tailored to their financial capacity.
aimClear: aimClear wants to be the single resource when it comes to your personal as well as company brand by using every possible tool in the marketer’s kitty – from social media to SEO, from conclaves to data management using basic tools like Excel.
Bufferapp social: Bufferapp is an exhaustive resource to help you manage, grow, and track your social media persona.
HubSpot Blog: A collection of 3 secondary blogs – focusing on sales, marketing, and your agency. This trio will help you streamline your sales activity, boost your pre-sales capabilities, help you traverse the maze of sales-killer questions like, “This is too costly”, and also help you manage your time and productivity in a better manner.
Marketing Land: The Dealbook of marketing, ML is the one-stop solution for your marketing trends news requirement.
Startup Marketing | Ryan Gum: Ryan Gum is a growth hacker who helps create marketing plans for startups and small businesses starting from creating your initial pitch, attracting your first customer, and an exhaustive list of Startup Marketing Resources.
The SumAll Blog: The SumAll blog is like the child of Buzzfeed and Medium (and not in a negative sense). They post content which is useful and is also written in a fun manner. The articles are often crisp and under a 5-min read.