If you're new to BusyCal, check out the list of BusyCal's top 10 features. If you're an existing BusyCal 1.6 user, here's a list of some of the new features and changes in BusyCal 2.0:

• Custom Month and Week Views — choose the number of weeks to show in the Month View (from 1-12 weeks), and the number of days to show in the Week View (from 2-14 days). Scroll the calendar incrementally or a page at a time, using the new navigation arrow buttons or from the keyboard.
• Year View — new Year View displays 12 mini-months with events & banners represented meaningfully.
• BusyCal Menu — new BusyCal menu displays the current weather, a list of today's events and to dos, and a Quick Entry field for creating new events with natural language. You can display the BusyCal Menu by clicking the menu bar icon, or by typing a shortcut key. You can even mark to dos as completed from the menu.
• Alarms — improved Alarm window lets you apply custom snooze intervals, including a new snooze until "x minutes before an event starts", view the name and color of the calendar the event is stored on, mark To Dos as completed within the alarm window, and navigate the alarm window from the keyboard. Or, you can optionally display alarms using Mountain Lion Notification Center.
• List View — improved List view lets you customize the view settings, including date range, columns shown, and sort order. You can even show a Total Duration column to calculate billable hours.
• Smart Filters — new Smart Filters are a powerful tool for creating calendar sets and saved searches that can be applied with a single click from the toolbar. You can create smart filters to display a set of calendars, show events that match certain conditions, and/or to apply view settings, such as the columns displayed and sort order in the List View.
• Find dialog — new Find dialog lets you search your entire calendar independent of the current view.
• Graphics palette — new Graphics palette lets you drag graphics onto your calendar. You can browse through hundreds of Emoji images built-in to OS X, or type a keyword to search for images on IconFinder.com. Plus improved UI for selecting, dragging, resizing, and minimizing graphics.
• Quick Entry — new Quick Entry feature lets you create events from natural language.
• To Do list — improved To Do List lets you choose custom date ranges, perform manual sub-sorting by dragging, and displays priority symbols.
• Weather — improved weather now includes a 10-day weather forecast from Wunderground.com, improved weather and moon graphics including a new background weather style, and auto-detection of your location via Location Services.
• Info Panel — improved Info Panel includes a new mini-month Date Picker for entering dates, ability to change date/time with arrow keys, and a new timestamp button for entering the current date and time in the Notes field.
• New Event defaults — new preferences for setting default calendars for new events & to dos, default start time & duration, and default alarms for new events, banners, to dos, and birthdays & anniversaries.
• Calendar Appearance — new Appearance preferences allow you to customize the day and time formating and font styles.
• Automatic Backups — improved Backup preferences allow you to set the location for the automatic backups.
• Birthdays & Anniversaries — option to display a Birthday and/or Anniversary calendar, including tiny gift and ring icons for each.
• Today color — enhanced the today highlight color to make it easier to see.
• Accounts — new Accounts pane for adding and editing CalDAV accounts, including the ability to change the account name displayed in the Calendar List.
• iCloud support — improved iCloud suppport, including calendar sharing support, meeting notification inbox, and optional warning when modifying shared events.

Requirements

BusyCal 2.0 requirements:

• OS X 10.8 Mountain Lion is required.
• An iCloud or Google Calendar account is recommended. Other CalDAV Servers are also supported (OS X Server, Kerio, Zimbra, Zarafa, Communigate, and Fruux).
• BusyCal syncs with with the OS X Calendar app through iCloud or Google Calendar (or another CalDAV server).
• BusyCal syncs with the iOS Calendar app on the iPhone through iCloud or Google Calendar (or another CalDAV server). BusyCal will not sync with an iPhone via iTunes.
• Sync Services is not supported. This means BusyCal will not sync local calendars with OS X Calendar or other third-party apps (like Outlook).
• Exchange is not supported.

For more info, see Upgrading from BusyCal 1.6 to BusyCal 2.0